Emergency notification systems, such as fire alarm systems, typically include one or more notification appliances for providing occupants of a building with a prominent visual or auditory indication of a hazardous condition, such as the presence of smoke or fire. A notification appliance circuit (NAC) connects the notification appliances to a central control panel, such as a fire alarm control panel. A primary power source, such as line power from an AC line, may supply power to the control panel. The NAC may thus provide power from the control panel to the notification appliances.
A notification appliance that is commonly employed in emergency notification systems is a strobe. The most common type of strobe is a Xenon flash tube based strobe. Light emitting diode (LED) strobes have recently been introduced into the marketplace and offer prospects of lower energy consumption. A notification system may include dozens, or even hundreds, of strobes distributed throughout a building. A first important consideration when designing a notification system that employs strobes is the energy efficiency of the strobes. It is generally preferable to maximize the number of strobes connected in series on a single NAC of a notification system in order to minimize wiring requirements and to reduce the overall cost of installing the notification system. It is generally also preferable to minimize the current requirements of a NAC in order to reduce energy consumption and operating costs. Employing strobes that operate more efficiently allows a greater number of strobes to be connected to a NAC at a lower current draw relative to strobes that operate less efficiently.
A second important consideration when designing a notification system that employs strobes is the ability of the strobes to deliver consistent light output, including consistent color and intensity, during operation. National Fire Protection Association (NFPA) requirements dictate that notification system strobe lights output a minimum total amount of light over a given time period for a given area. A strobe that lacks consistency and produces varying levels of light output from flash to flash may cumulatively project too little light over a given time period and thus fail to meet the NFPA output requirement. Conversely, an inconsistent strobe may produce a cumulative amount of light over a given time period that greatly exceeds the NFPA output requirement, thereby compromising the efficiency of the NAC. Consistent light output is therefore critical for ensuring compliance with NFPA requirements while optimizing system efficiency.
Referring to FIG. 1, a schematic diagram of a conventional drive circuit 10 for an LED strobe application is shown. The drive circuit 10 includes a buck convertor or boost convertor 12 for stepping down or stepping up a NAC input voltage, respectively, a current limiting resistor 14, a LED 16, and a transistor switch 18 for flashing the LED 16. While generally effective for providing a strobe, this configuration exhibits certain inefficiencies and can cause significant variations in light output among a group of serially-connected LED strobe units. For example, with regard to efficiency, an embodiment of the drive circuit 10 that employs a buck convertor at 12 requires a minimum convertor input voltage that is greater than the convertor's regulated output voltage which is greater than the stack up voltage of the LED 16 and other drive circuit elements. Alternatively, an embodiment of the drive circuit 10 that employs a boost convertor at 12 requires a minimum input voltage for facilitating a defined duty cycle for proper boost operation. The output voltage of the boost convertor 12 must also be greater than the stack up voltage of the LED 16 and other drive circuit elements. Thus, for either embodiment of the drive circuit 10, a substantial amount of energy is wasted during operation.
With regard to light output, the tolerances of the energy source voltage, the current limiting resistor 14, the LED's forward voltage drop, and the voltage drop across the switch element 18 of the drive circuit 10 can all affect the amount of current passing through the LED 16, thereby diminishing the consistency of the LED's output. While the tolerances of the energy source components, limiting resistor 14, and switch element 18 can be narrowed by implementing components with tighter tolerances, the forward voltage drop across the LED 16 can nonetheless vary by 15% or more from the effects of drive current, duty cycle, thermal resistance, and ambient temperature on the LED's junction. LED output may therefore be highly inconsistent and may vary in color and intensity during strobe operation.